The subject matter of the present invention relates to a shock sub, hereinafter known as a "weight on bit equalizer", adapted for use with a wellbore drilling tool, the drilling tool being used for drilling a hole in an earth formation known as a wellbore. More particularly, the present invention relates to a shock sub, also known as a weight on bit equalizer apparatus, adapted for use with a wellbore drilling tool, the shock sub providing a constant weight or force on a drilling bit of the wellbore drilling tool when the drilling bit is drilling a wellbore. The constant force is provided by a precharged nitrogen chamber adapted for telescopically opening the weight on bit equalizer apparatus and providing a weight or force on the drilling bit. However, an additional differential force is applied to the drilling bit; therefore, the shock sub includes a pressure compensation apparatus adapted for cancelling out the additional differential force being applied to the drilling bit thereby providing a constant weight or force on the drilling bit.
When a drilling tool is disposed in a wellbore, a drill bit on the drilling tool will contact an earth formation and a rotation of the drill bit will cause the drill bit to drill a hole, known as a wellbore, in the earth formation. When the drill bit contacts the formation during the drilling operation, it is optimum if the weight or force of the drill bit on the formation is approximately constant during the drilling operation. However, certain environmental factors exist within the wellbore, during the drilling operation, which tend to change the weight or force being applied to the drill bit. Therefore, the instantaneous weight or force of the drill bit on the formation will change from moment to moment during the drilling operation and this change will prevent the weight or force of the drill bit on the formation from being constant during the drilling operation. For example, when the drilling tool is temporarily stuck in the wellbore, a stick slip situation may tend to momentarily change the weight or force of the drill bit on the formation during the drilling operation. For example, mud with a high positive differential overbalance pressure can cause differential sticking which can cause friction that leads to the stick slip situation. Since a constant weight or force of the drill bit on the formation is desired, it is therefore necessary to build the drilling tool in a particular manner such that the adverse influence of the wellbore fluid on the drill bit is cancelled out during the drilling operation.